The invention concerns an SiO2-glass bulb with at least one current lead-in made of a gas-tight composite material, such that the composite material consists of a noble metal with a melting point  greater than 1,700xc2x0 C. and SiO2 and is at least partially coated with a layer of SiO2. The invention also concerns a high-intensity discharge lamp and a process for producing a gas-tight connection between an SiO2-glass bulb and a current lead-in.
Metallic or composite current lead-ins for SiO2-glass bulbs are well known. The term composite is understood to mean a combination of different types of materials. In the present case, we are concerned, specifically, with a combination of a glass material and a metallic material. In the formation of a gas-tight connection between the material SiO2 and an electrically conducting, metallic or metal-containing current lead-in, it is necessary to deal with the basic problem that the metal components of the current lead-in are poorly wetted by viscous SiO2. In addition, the low coefficient of thermal expansion of SiO2 compared to that of a metal makes it difficult to form a gas-tight connection. During the cooling process after sealing, the metallic or metal-containing current lead-in contracts more strongly than the SiO2 of the glass bulb, so that there is a tendency for a gap to form at the interface between the glass bulb and the current lead-in. Although this risk can be reduced by minimizing the thickness of the current lead-in, it is difficult to position and handle very thin current lead-ins, e.g., in the form of foil. To be able to produce a gas-tight connection despite these problems, only relatively expensive solutions have been proposed so far.
For example, EP 0,938,126 A1 describes a current lead-in made of a composite material for a lamp, especially a discharge lamp, in which the composite material consists of SiO2 and metal, and in which the metal content changes along the length of the current lead-in. The metal content can vary from 0 to 100%. The end with the low molybdenum content is directed towards the discharge space of the lamp and is connected with the lamp bulb in a gas-tight connection. Only the front end of the current lead-in, which consists mainly or entirely of SiO2, is in direct contact with the gas in the discharge space. A metallic electrode mount is sintered into the current lead-in on the end with the low metal content. This mount is inserted deep enough into the current lead-in to produce direct contact with a composite region in which the SiO2 content is xe2x89xa780%. This produces an electrical contact between the electrode mount and the metal-rich end of the current lead-in. The composite material disclosed in the cited document consists of a metal powder that consists of molybdenum with an average particle size d50 of 1 xcexcm and a glass powder with an average particle size d50 of 5.6 xcexcm.
EP 0,930,639 A1 likewise discloses a current lead-in with a metal content that changes along its length and an SiO2 lamp bulb. Metals that are specified as suitable for the composite material include not only molybdenum, but also tungsten, platinum, nickel, tantalum, and zirconium. To protect the metal-rich end of the current lead-in from oxidation, a protective coating of glass, metal oxide, noble metal, or chromium is provided, which partially covers the part of the current lead-in that extends out of the lamp bulb. The gas-tight seal between the current lead-in and the lamp bulb is located in a region of the current lead-in in which the concentration of the metal in the composite material is less than 2%.
However, the production of a current lead-in with a metal concentration that changes along the length of the current lead-in requires expensive equipment. Different powders must be produced and arranged in layers. In addition, when an electrode is being sealed into the current lead-in, it is necessary to consider the electrical conductivity of the individual layers and thus the depth of insertion of the electrode in the current lead-in in order to produce a solid electrical contact. To be able to achieve a gas-tight connection, the sealing with the SiO2 lamp bulb must be performed in a specific segment of the length of the current lead-in with a very low metal concentration. Furthermore, at high temperatures in the region of the current lead-in, corrosion can occur in metals that are not resistant to oxidation, such as molybdenum.
EP 0,074,507 A2 describes a material for electrical contacts, especially light-duty contacts, and a process for producing it. The material consists of a noble metal with 1 to 50 vol. % of glass, in which a noble metal powder with a particle size of xe2x89xa6250 xcexcm and a glass powder with an average particle size of xe2x89xa650 xcexcm are preferably used. Gold, silver, palladium and their alloys are used as the noble metals.
The object of the present invention is to provide a gas-tight, corrosion-resistant current lead-in for an SiO2-glass bulb, preferably a discharge lamp, which has high electrical conductivity and is easy to produce and handle.
Pursuant to this object, and others which will become apparent hereafter, one aspect of the present invention resides in the noble metal and the SiO2 being homogeneously distributed in the composite material. The noble metal content of the composite material is xe2x89xa710 vol. % to xe2x89xa650 vol. %, and the SiO2 coating covers the composite material at least in the region of the connection with the SiO2-glass bulb.
The SiO2 used to produce the composite material should have a purity of xe2x89xa797 wt. %. Accordingly, impurities in the SiO2, e.g., alkali metals or alkaline-earth metals, can be tolerated up to ca. 3 wt. %.
Due to the SiO2 coating, the current lead-in can be sealed gas-tight with the SiO2-glass bulb along its entire length or along any desired segment of this length. Only a single composite powder is needed to produce the current lead-in. Since the current lead-in shows uniformly high electrical conductivity along its entire length, when an electrode is sealed into the current lead-in, it is not necessary to consider its depth of penetration into the composite material. The proportion of noble metal in the current lead-in can be used to adjust the coefficient of thermal expansion, which is preferably selected in the range of  less than 5xc2x710xe2x88x926 l/K for the current lead-in. The current lead-in of the invention has the especially advantageous property that the SiO2-containing composite material of which it is made, which has a noble metal content of xe2x89xa710 vol. % to xe2x89xa650 vol. %, is readily deformable at temperatures greater than about 1,200xc2x0 C. At temperatures greater than about 1,600xc2x0 C., current lead-ins designed, for example, in the form of rods bend under their own weight to an angle of 90xc2x0 without developing cracks and without impairing the electrical conductivity of the material. This property makes it possible to straighten and align a current lead-in of this type.
To be sure, these mechanical properties are similar to those of pure quartz glass, but it is surprising that they are also found in the composite material with its very high electrical conductivity and current-carrying capacity. A measured current-carrying capacity of 20 amperes in a rod of composite material with a diameter of 2 mm indicates a cohesive network of the noble metal component, which would normally be rigid and hardly deformable. These properties of the composite material, which are a combination of the deformation properties of the pure quartz glass and the conductivity of the noble metal, allow precise and very easy fitting of electrodes or contact pins to the current lead-in. For example, a tungsten electrode can be fastened to the end of the current lead-in, which points towards the inside of the glass bulb, by heating the electrode together with the powder mixture. It is also possible to sinter the electrode into composite material that has already been formed. In addition, an electrode can be inserted into viscous composite material that has been heated to about 1,200xc2x0 C. In all three cases, a sufficiently conductive electrical connection is produced in a very simple fashion. A contact pin can be connected with the end of the current lead-in that is directed away from the glass bulb in the same way. The electrode or contact pin can also be aligned, i.e. its position or location can be corrected, or the straitness of the current lead-in itself can be corrected at temperatures of about 1,200xc2x0 C.
The composite material is preferably formed by heating a powder mixture of noble metal powder and SiO2-glass powder. The noble metal may also be a noble metal alloy. The noble metals platinum, rhodium, ruthenium, rhenium, and iridium have been found to be especially suitable for use in the composite material. The electrical conductivity of the current lead-in is preferably selected in the range of  less than 0.01 m/xcexa9mm2. The thickness of the SiO2 coating should be 5-25 xcexcm and especially 7-15 xcexcm. A noble metal powder with a BET (Brunauer-Emmett-Teller) specific surface of 0.01 to 10 m2/g is especially suitable. It is also advantageous to use a noble metal powder with an average particle size (d50) of 3 to 30 xcexcm. The SiO2-glass powder preferably has a BET specific surface of 10 to 100 m2/g. An average particle size (d50) of the SiO2-glass powder of 0.1 to 10 xcexcm has been found to be advantageous. It is especially cost-effective if the noble metal component of the composite material is present in amounts of only 10 vol. % to 25 vol. %.
The use of the SiO2-glass bulb and current lead-in of the invention is ideal for high-intensity discharge lamps due to the excellent corrosion resistance, high conductivity, and high level of gas-tightness of the lead-in.
The goal of the invention with respect to a process for producing a gas-tight connection is achieved with a process in which the powder mixture is heated to a maximum of 1,200-1,600xc2x0 C. After the material has been heated, the layer of SiO2 is applied to the gas-tight composite material in the region of the connection with the SiO2-glass bulb. The current lead-in is inserted into an opening in the SiO2-glass bulb, and the current lead-in is sealed gas-tight with the SiO2-glass bulb in the region of the SiO2 coating at a temperature  greater than 1,600xc2x0 C. The SiO2 coating is preferably applied to the composite material in the form of a paste or a suspension by spraying, printing, or dipping, after which the SiO2 coating should be fired on the composite material. However, the SiO2 coating may also be applied to the composite material by vacuum evaporation, sputtering, chemical deposition, or thermal spraying.
When the noble metals ruthenium and/or rhenium and/or iridium are used for the composite material, the goal of the invention with respect to a process for producing a gas-tight connection is also achieved with a process in which the powder mixture is heated to a maximum of 1,200-1,600xc2x0 C. After it has been heated, the gas-tight composite material is at least partially calcined in an oxygen-containing atmosphere at a temperature xe2x89xa71,600xc2x0 C., so that the noble metal on the surface of the composite material is oxidized and vaporized, and a layer of SiO2 is produced at least in the region of the connection with the SiO2-glass bulb of the lamp. The current lead-in is inserted into an opening in the SiO2-glass bulb, and the current lead-in is sealed gas-tight with the SiO2-glass bulb in the region of the SiO2 coating at a temperature  greater than 1,600xc2x0 C.
This process exploits the fact that the metals ruthenium, rhenium, and iridium, which form volatile oxides, are oxidized and vaporized at the surface of the composite material, when the composite material is heated to a temperature xe2x89xa71,600xc2x0 C. in an atmosphere that contains oxygen. During the calcining process, a thin, closed layer of SiO2 forms around the composite material and prevents further volatilization of the metal. This layer of SiO2 can then be satisfactorily sealed gas-tight with the SiO2 of the glass capsule. The seal is so stable mechanically that an atomic bond is probably formed between the SiO2 of the glass capsule, the SiO2 coating produced by the calcining, and the SiO2 in the composite material.
Air is preferably used as the oxygen-containing atmosphere, but it is also possible to use pure oxygen or other gas mixtures that contain oxygen.
It is especially advantageous to increase the temperature incrementally to a maximum of 1,200-1,600xc2x0 C. during the heating of the powder mixture.
A process in which the powder mixture is shaped before being heated is cost-effective. It was found to be effective to shape the power mixture by stamping or extrusion before heating it. If an unshaped powder mixture is heated (which, of course, is also possible), the composite material produced in this way must then be shaped. However, due to the high strength of the composite material, this can generally be accomplished only by machining methods, which are less cost-effective.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of the disclosure. For a better understanding of the invention, its operating advantages, and specific objects attained by its use, reference should be had to the drawing and descriptive matter in which there are illustrated and described preferred embodiments of the invention.